1. Field of the Invention
The present invention relates to a method for correcting the positioning error of an actuator having a wave gear reduction drive, and to an actuator drive control apparatus that uses the method in the actuator positioning operation.
2. Description of the Prior Art
There are actuators that are configured to move an object to a target position in which the speed of the rotational output of a motor is reduced by means of a wave gear reduction drive. The wave gear reduction drive comprises a rigid, circular spline gear with internal teeth, a flexible, circular, externally toothed spline gear (flexspline), and a wave generator. In a typical configuration, the wave generator flexes the flexspline into an elliptical shape, causing the toothed portions of the flexspline at each end of the elliptical major-axis to mesh with the corresponding internal teeth of the rigid circular spline. When the wave generator is rotated by a motor, the positions at which the two splines mesh move around the circumference, generating a relative rotation that corresponds to the difference in the number of teeth in the circular spline and in the flexspline (2n teeth, where n=1, 2, 3 . . . ). Normally, the difference in the number of teeth in the circular spline and in the flexspline is two. With the rigid circular spline fixed, the flexspline functions as a rotating speed-reduction output element whereby, on the load side to which the flexspline is coupled, the low-speed rotation is used to drive an object. The speed reduction ratio i is found by i=1/R=(Zcxe2x88x92Zf)/Zf, where R is the speed ratio, Zf is the number of teeth in the flexspline, and Zc is the number of teeth in the rigid circular spline. If Zf=100 and Zc=102, for example, then the speed reduction ratio i would be {fraction (1/50)}, with the output rotation being in the reverse direction to the direction of motor rotation.
Drive control systems of actuators thus configured normally apply feedback control to control actuator positioning. However, wave gear reduction drives suffer from angular transmission error, giving rise to error between the actual point at which the actuator output shaft (the output shaft of the wave gear reduction drive) is positioned and the target position. If this error could be compensated for, it would increase the positioning accuracy of actuators equipped with wave gear reduction drives.
In the case of a two-tooth difference between the two splines, if R is the speed ratio of the wave gear reduction drive, the positioning error component per output shaft revolution can be divided into
(1) Period R error component
(2) Period (R+1) error component
(3) Other error components.
Period R error components are an attribute of the rigid circular spline and (R+1) error components are an attribute of the flexspline.
An object of the present invention is to provide a method of correcting positioning error in a drive control apparatus of an actuator equipped with a wave gear reduction drive by adding, to the positioning feedback control loop thereof, a position correction function for correcting the above period (R+1) positioning error arising in the angular transmission error of the wave gear reduction drive to thereby improve the accuracy of the drive control apparatus.
In accordance with the present invention, the above object is attained by a method of correcting positional error in an actuator having a motor and a wave gear reduction drive that transmits a rotational output of the motor to a load at a reduced speed, said method comprising:
measuring positioning error relative to an absolute position of an actuator output shaft to produce error correction data relating to a rotational position of the motor shaft;
detecting a rotational position of the motor shaft;
producing rotational position correction information by adding an error correction value in the error correction data to the detected rotational position, the error correction value being assigned to the detected rotational position in the error correction data; and
using the produced rotational position correction information as position feedback information for controlling the positioning of the output shaft;
wherein the error correction data is produced by measuring the positioning error for at least 1(R+1) revolution of the output shaft, where R is a speed ratio of the wave gear reduction drive.
The above object is also attained by an actuator drive control apparatus in which the above method is used to correct positioning error, said apparatus comprising:
an error correction data memory section in which error correction data is stored:
an output shaft starting-point sensor for detecting an absolute position of an output shaft:
a position detector for detecting a rotational position of a motor shaft:
a rotational position correction information generator for producing rotational position correction information based on rotational position information supplied by the output shaft starting-point sensor and the position detector, and on the error correction data: and,
a feedback control section that effects feedback control by using the rotational position correction information as positional feedback information to position the actuator at a target position indicated by positional command information.
The error correction data can be maintained in memory in the position detector, in which case the data can be downloaded from the position detector to the error correction data memory section after the system power has been switched on. The error correction data can be in the form of a correction pulse data sequence or an approximation coefficient sequence representing the error correction information.